All the heretofore-known applanation tonometers may be subdivided into tonometers featuring a constant pressure applied to the corneal surface to flatten it, and those featuring a variable force applied to the corneal surface with the same purpose. In the former-type tonometers a measure of the intraocular pressure is the diameter of an applanation circle which is meant as an averaged diameter of an impression or indentation corresponding to a flattened area of the corneal surface resulting from the effect of a known constant force applied to the cornea, e.g., gravitational force. Most commonly used tonometers of that type are such as, e.g., Maklakov's Filatov-Kalfa's (cf. A. Nesterov et al. "Intraocular pressure (physiology and pathology)", Mir Publishers, Moscow, 1978, pp. 21-22).
However, the aforementioned tonometers suffer from too sophisticated calibration and low measuring accuracy due to an adverse effect of the corneal rigidity forces and of a ring of the lacrimal fluid arising round the applanation circle.
The latter-type tonometers may be exemplified by those of Goldmann's, Mackey's, Marg's, Draeger's (cf. A. Nesterov et al., op. cit., pp. 21-23).
In these tonometers a measure of the intraocular pressure is a force exerted on the cornea to form an applanation circle thereon having a diameter equal to 3.06 mm, or appearance of a dicrotic wave on the curve characteristic of the force applied versus time, i.e., an abrupt change in the recorded curve representing the force applied. Studies performed demonstrate that the data obtained from measurement of intraocular pressure with the aid of Mackey-Marg tonometer agree with those obtained with the aid of Goldmann's or Draeger's tonometer, the latter being a portable modification of the former tonometer. However, the Mackey-Marg tonometer suffers from the fact that its measuring circuit is too complicated in manufacture due to a stringent requirement imposed on it as to high sensitivity thereof.
When using a Goldmann's tonometer, the instant when the diameter of the applanation circle becomes equal to 3.06 mm is registered visually and the force of pressure exerted on the corneal surface is measured at that instant.
In this case the surface tension forces and the corneal rigidity forces are mutually balanced, therefore the force applied to the applanation area from the side of the tonometer is equal to a true intraocular pressure. The measurement accuracy is increased due to use of a corneal microscope and fluoroscein with which lacrimal fluid is stained.
However, necessity for use of fluorescein complicates operation with Goldmann's tonometer.
Besides, the aforelisted tonometers of the second type are adapted largely for measuring intraocular pressure alone and their application for measuring the hydrodynamical coefficients C and F is rather difficult.
One more applanation tonometer is known to comprise a housing, a rod accommodated in the housing and carrying a contact disk at one of its ends, one of the surfaces of the disk, i.e., a working surface, being adapted to interact with the cornea and to exert a force thereon, a unit for measuring the force applied, associated with the rod, a means for registering the instant when the diameter of an applanation circle on the corneal surface, formed due to the effect of a force exerted by the contact disk on the cornea, reaches a preset value, and a recording unit connected to the outputs of the measuring unit and of the registering means (cf. a prospectus of Bausch Lomb Co, Mark of leadership, "The non-gravimetric tonometer", 1981).
Intraocular pressure is measured with the aid of this tonometer according to the same technique as is the case with Goldmann's tonometer, i.e., there is measured the force of pressure exerted by the contact disk on the cornea surface at the instant when the diameter of the applanation circle on the corneal surface becomes equal to approximately 3.06 mm.
The means for registering the instant when the diameter of an applanation circle reaches a preset value incorporates a system of holes arranged along the periphery of the contact disk, a flexible envelope interposed between the disk working surface and the cornea, a passage for gas to feed from a gas-containing cylinder, said passage being situated inside the rod. The gas pressure in the system is measured with a pressure gauge that generates an electric signal which is applied to the recording unit made as a recorder.
However, the aforesaid known tonometer requires, for the purpose of an adequately high measuring accuracy, highly accurate manufacture of the components of the registering means which is pneumatic as for its operating principle and features a sophisticated construction. This results in more complicated production of the components of the registering means. In addition, the necessity for use of an inert gas (of the type of freon) free from solids also adds to the difficulties involved in operation of the instrument.